1. Field of the Invention:
The present invention relates to an improvement of oxygen sensor elements, and more particularly the invention relates to oxygen sensor elements having an electrode composition which especially possesses improved gas permeability, gas response characteristics, and durability.
2. Related Art Statement:
Heretofore, oxygen sensors have been used for measuring concentrations of oxygen in exhaust gases from internal combustion engines or the like on the principle of an oxygen concentration cell with the use of an oxygen ion conductive solid electrolyte.
Such oxygen sensors are ordinarily constituted such that a cylindrical bottom-closed shape of an yttria-added zirconia ceramic or the like is used as an oxygen ion conductive solid electrolyte, e.g., electrodes made of platinum are formed on the inner and outer surfaces of the solid electrolyte, and the inner surface electrode is communicated, as a reference oxygen concentration electrode, with open air while the outer surface electrode is exposed, as a measuring electrode, to exhaust gases to be measured. Thus, the concentration of oxygen in the exhaust gases can be measured based on output voltages from the oxygen sensor.
However, electrodes of such an oxygen sensor element to be used in the oxygen sensor of this type are formed by, for example, chemical plating, or physical vapor depositing, in a filmy fashion in a thickness of as thin as several .mu.m. Therefore, the electrodes have poor gas permeability and bad gas response . In order to improve the response characteristics, heating is employed. However, such a heating technique has problems that the electrodes made of platinum or the like are sintered, catalytic activity drops, adhesion of the electrodes to the substrate becomes poor, the number of three phase points mentioned later decreases, and low temperature operability is deteriorated.
Recently, it has become necessary that oxygen sensors withstand several uses, and possess longer use life under more severe conditions than before. It is desired that electrodes to be used in such oxygen sensors have high gas permeability leading to response characteristic, and greater surface area and high catalytic activity leading to low temperature operability, and that these characteristics are not deteriorated; that is, the electrodes have excellent durability. Furthermore, since conventional electrodes are easily sintered or abraded during exposure to high temperature exhaust gases in their operation, their catalytic activities are damaged, and electric conductivity is deteriorated. Thus, such electrodes cannot satisfy the above-mentioned requisites.
On the other hand, in order to improve durability (adhesion), Japanese Patent Publication No. 45-30,617 discloses an oxygen sensor element in which a porous supporting layer is provided in a thickness of around 100 .mu.m between a solid electrolyte and an electrode.
However, although this electrode has good durability due to its increased layer thickness, the electrode has the shortcoming that its response characteristic becomes poor because penetrating or exchanging of a gas to be measured through the electrode layer takes a long time. For this reason, the thickness of such an electrode must generally be limited to around several .mu.m so that sufficient gas permeability or response characteristic may be obtained.
However, since the electrode having such a thickness is filmy, the oxygen sensor element has the problem that the gas permeability thereof or its response characteristic is not so excellent, and that when actually exposed to high temperature exhaust gases, the electrode is likely to be sintered.